i am not shore abut this qustion .because its not easy . but i read on book writer Albert Flack was Professor of Islamic finance and I trade currencies and cryptocurrencies.
1 As about Bitcoin ,with the lenght of the process of the settlement, it is no more seen as a payment tool, yet, but it is seen as a warehouse of value ( an asset) , if you want like a Diamond. It is rare and people do not trade diamonds for buying a coffee. So it is a digital asset. It is not seen as a currency but it is seen like a digital gold. No problem. Gold Silver is one of the 7 islamic finance currencies, you buy it cash , you sell it cash. Bitcoin are traded cash .
(a) For the other currencies you may consider they are still payment tools because they have cheaper value and some like Ripple etc are indeed in the creation of a system of payment. Once again there are cash payments.
(b) what is HARAM not authorized, is to buy more then what you can pay cash , or to sell short what you do not have. Because it enters in the notion of speculation and hazard, you may loose more then your money , or you may enter in a systemic risk. If you loose your money ok you lost it, but if you loose and can not settle debts then your problem become the problem of your lender who may also not settle his debts in a cascade of systemic problem, harming the community and the system.
(2)What is also Halal is the notion of Property under construction : I explain. If you know that end of the month you will receive your rents, your retirement, your reimbursements, or some money lets say inheritage. you can buy in advance borrowing money now what will arrive to you later on. Lets now exagerate and say that you have a flat and that you now you will receive the next 12 months of rent or lets say you have a field of petrole, producing 100 barrils a day, 36500 barrils a year. You can sell the petrole 36500 barrils ( why not 10 years also), and OR borrow money , for this money and invest in cryptocurrencies because you know that these future incomes or flows will arrive to you. So you can buy in advance ( with credit and not cash) the future flows that you know will arrive to you